Description

The mq_receive() function receives the oldest of the highest priority message(s) from the message queue specified by mqdes. If the size of the buffer in bytes, specified by msg_len, is less than the mq_msgsize member
of the message queue, the function fails and returns an error. Otherwise, the selected message is removed from the queue and copied to the buffer pointed to by msg_ptr.

If the value of msg_len is greater than {SSIZE_MAX}, the result is implementation-defined.

If msg_prio is not NULL, the priority of the selected message is stored in the location referenced by msg_prio.

If the specified message queue is empty and O_NONBLOCK is not set in the message queue description associated with mqdes, (see mq_open(3C) and mq_setattr(3C)), mq_receive() blocks, waiting until a message is enqueued on the message queue, or until mq_receive() is interrupted by a signal. If more than one process (or thread) is waiting to receive a message when a message arrives at an empty queue, then the process of highest priority that has been waiting the longest is selected to receive the message. If the specified message queue
is empty and O_NONBLOCK is set in the message queue description associated with mqdes, no message is removed from the queue, and mq_receive() returns an error.

The mq_timedreceive() function receives the oldest of the highest priority messages from the message queue specified by mqdes as described for the mq_receive() function. However, if O_NONBLOCK was not specified
when the message queue was opened with the mq_open(3C) function, and no message exists on the queue to satisfy the receive, the wait for such a message is terminated
when the specified timeout expires. If O_NONBLOCK is set, this function is equivalent to mq_receive().

The mq_reltimedreceive_np() function is identical to the mq_timedreceive() function, except that the timeout is specified as a relative time interval.

For mq_timedreceive(), the timeout expires when the absolute time specified by abs_timeout passes, as measured by the CLOCK_REALTIME clock (that is, when the value of that clock equals or exceeds abs_timeout),
or if the absolute time specified by abs_timeout has already been passed at the time of the call.

For mq_reltimedreceive_np(), the timeout expires when the time interval specified by rel_timeout passes, as measured by the CLOCK_REALTIME clock, or if the time interval specified by rel_timeout is negative
at the time of the call.

The resolution of the timeout is the resolution of the CLOCK_REALTIME clock. The timespec argument is defined in the <time.h> header.

Under no circumstance does the operation fail with a timeout if a message can be removed from the message queue immediately. The validity of the timeout parameter need not be checked if a message can be removed from the message queue immediately.

Return Values

Upon successful completion, mq_receive(), mq_timedreceive(), and mq_reltimedreceive_np() return the length of the selected message in bytes and the message is removed from the queue. Otherwise, no message is removed from the queue,
the functions return a value of -1, and sets errno to indicate the error condition.

Errors

The mq_receive(), mq_timedreceive(), and mq_reltimedreceive_np() functions will fail if:

EAGAIN

O_NONBLOCK was set in the message description associated with mqdes, and the specified message queue is empty.

EBADF

The mqdes argument is not a valid message queue descriptor open for reading.

EINTR

The function was interrupted by a signal.

EINVAL

The process or thread would have blocked, and the timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1,000 million.

EMSGSIZE

The specified message buffer size, msg_len, is less than the message size member of the message queue.

ETIMEDOUT

The O_NONBLOCK flag was not set when the message queue was opened, but no message arrived on the queue before the specified timeout expired.

The mq_receive(), mq_timedreceive(), and mq_reltimedreceive_np() functions may fail if: